Directional valves are known per se; they allow the flow of a particular medium, for example, a gas stream, in a flow direction determined by the design of the directional valve, and block the flow of the particular medium if the flow arrives against the flow direction.
A directional valve acting as an exhalation valve of a breathing mask is known from DE 1 027 518 A. This directional valve comprises a valve lower part with a valve seat and with a centrally held closing element. It was observed at that time as a drawback of prior-art exhalation valves that the pressure difference, which is to be overcome during the exhalation, increases approximately proportionally with the flow velocity and hence also with the quantity of air exhaled per unit of time. This causes the exhalation resistance to increase excessively in a physiologically unfavorable manner during hard work and consequently deep breathing. An elastically deformable, frustoconical closing element was proposed at that time to avoid this drawback. It should, however, be noted in connection with this solution that only a part of the cross-sectional area is released by the closing element in the flow direction and that if such a directional valve is placed in a tube, for example, an exhalation tube, the gas stream through the closing element is deflected towards the tube wall, which results in an unfavorably increased flow resistance.
Further directional valves with elastically deformable closing elements have meanwhile become known.
For example, DE 10 2010 008 923 A shows a directional valve, in which two elastic flaps arranged at the edge in a valve housing act as the closing element and a middle web on the valve housing and support webs arranged on both sides in relation thereto act as a valve seat.
A directional valve, which opens and closes according to the principle of a heart valve, is known from WO 2012/010815 A1. It has, however, the drawback that the individual valve parts cannot be laid down on a support geometry, so that the valve parts may turn over under an increased back pressure and the valve will consequently fail to close correctly. In addition, the individual valve parts must be manufactured with extremely high precision in order to make it possible to guarantee tightness in the blocked direction under usual pressure conditions.